16 PROFESSOR A. M. MARSHALL. 



The large size of the eggs of Elasmobranchs, and perhaps that of 

 Cephalopods also, may possibly be associated with the carnivorous 

 habits of the animals ; for it is of importance that forms which prey on 

 other animals should hatch of considerable size and strength. 



The influence of habitat must also be considered. It has long been 

 noticed as a general rule that marine animals lay small eggs, while 

 their fresh-water allies have eggs of much larger size. The eggs of the 

 salmon or trout are much larger than those of the cod or herring ; and 

 the crayfish, though only a quarter the length of a lobster, lay eggs of 

 actually larger size. 



This larger size of the eggs of fresh-water forms appears to be 

 dependent on the nature of the environment to which they are 

 exposed. Considering the geological instability of the land as com- 

 pared with the ocean, there can be no doubt that the fresh-water fauna 

 is, speaking generally, derived from the marine fauna ; and the great 

 problem with regard to fresh-water life is to explain why it is that so 

 many groups of animals which flourish abundantly in the sea should 

 have failed to establish themselves in fresh water. Sponges and 

 Coelenterates abound in the sea, but their fresh-water representatives 

 are extremely few in number ; Echinoderms are exclusively marine ; 

 there are no fresh-water Cephalopods, and no Ascidians ; and of the 

 smaller groups of Worms, Molluscs, and Crustaceans, there are many 

 that do not occur in fresh water. 



Direct experiment has shown that in many cases this distribution is 

 not due to inability of the adult animals to live in fresh water ; and 

 the real explanation appears to be that the early larval stages are 

 unable to establish themselves under such conditions. This interesting 

 suggestion, which has been worked out in detail by Professor Sollas * 

 undoubtedly affords an important clue. To establish itself permanently 

 in fresh water an animal must either be fixed, or else be strong enough 

 to withstand and make headway against the currents of the streams or 

 rivers it inhabits, for otherwise it will in the long run be swept out to 

 sea, and this consideration applies to larval forms equally with adults. 



The majority of marine Invertebrates leave the egg as minute 

 ciliated larvae : and such larvse are quite incapable of holding their 

 own in currents of any strength. Hence, it is only forms which have 



1 ,W. J. Sollas, ' On the Origin of Freshwater Faunas.' Scientific 

 Transactions of the Royal Dublin Society, vol. iii. Ser. 11, 1886. 



